White light emitting diodes (LEDs) are commonly used for illumination or as flash lights, e.g., in mobile phones, digital still cameras, video cameras, or toys.
In one type of white LED, referred to as phosphor converted or phosphor coated LED (pc-LED), yellow emitting phosphor is placed on top of a blue LED to convert a fraction of the blue light to yellow light, which, in combination, renders white light. However, such LEDs exhibit a yellow appearance in their off-state, i.e., when no current is passed through the LED, due to the absorption of blue light by the phosphor. Such a yellow appearance is not desired by the customer.
In order to reduce this yellow appearance, white diffusers can be placed on top of the pc-LED. However, such diffusers reduce the efficiency of the LED, as a fraction of the emitted light is reflected back into the LED where it is absorbed.
The yellow appearance can also be reduced by placing electrically switchable polymer dispersed liquid crystals (PDLCs) on top of the pc-LED. However, the efficiency of a PDLC for hiding the yellow appearance is limited by the PDLC's relatively low back scattering.
WO 2008/044171 A2 discloses a lighting device comprising a pc-LED, the lighting device being configured for providing a residual current to the pc-LED in the functional off-state of the lighting device. The residual current is adjusted such that the pc-LED emits sufficient light to render a white appearance of the lighting device.